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LM5109A: When HI is always 5V, will HO keep high always?

Amelie Zheng
Genius 10865 points
Part Number: LM5109A

Hi Team,

If HI is 5V always, will HO keep high level always?

I just want to know if HI is always high, how could HO bootstrap?

  • 0
    TI__Expert 4070 points

    Hi Amelie,

    Thanks for reaching out!

    If you hold HI to logic high, the HO pin will initially output the sufficient V_GS to turn on the high-side FET, as supplied by the bootstrap capacitor that is referenced to the switching node.  BUT, as you suggested, as you continue to hold HI to logic high, the bootstrap capacitor will discharge and HO will have insufficient V_GS to keep the high-side FET on.  HO will decay to a diode drop below V_DD (referenced to GND)... and with an undefined switching node voltage (likely above GND), the V_GS on the high-side FET will be insufficient to turn it on. 

    I hope this was clear and answered your question, if so, please click the green button!  If you have further questions, please do not hesitate to continue the thread!

    Thanks!

    Aaron Grgurich